The present invention concerns thermal imaging, and more specifically concerns improved pixels or cells for arrays of bolometers.
Night vision and related applications can be realized by receiving the infrared radiation emitted by warm bodies in an array of detectors whose electrical output signals are converted into a visible image. The individual detectors, called cells, elements, or pixels, must be very small. They should also be sensitive to radiation over a wide band of the infrared spectrum, have minimal noise at ambient temperatures, operate at high sensitivity, and be inexpensive to manufacture.
Arrays of uncooled bolometers on a semiconductor substrate offer a promising approach to room-temperature infrared imaging. Such arrays also have shortcomings in several different respects. Wide bandwidth is difficult to achieve, and is sensitive to a large number of interacting factors in the structure and materials of the individual pixels. Noise can swamp the useful signals when the array is not cooled to cryogenic temperatures. Good sensitivity to minute temperature differences requires high absorption of incident radiation, and this is frequently detrimental to other design goals. While nanotechnology techniques have been employed for constructing bolometer arrays, the expense of meeting the operational goals can lead to high fabrication costs and low yields, especially for arrays having large numbers of individual image elements or pixels.
An array of bolometers according to the invention has a wide bandwidth in the infrared spectrum for increasing the range of temperatures that contribute to an image. It has a high absorption for increased sensitivity to temperature differences. Its structure, materials, and manufacture balance tradeoffs between competing factors in a way that reduces the cost of fabrication.
These and other goals are achieved in an uncooled array of thin bolometer pixels having a highly isolated platform structure of absorbing material separated from a substrate by a predetermined gap. A reflecting layer can increase efficiency and bandwidth. The array can also include a shaped resistor of material having a relatively low total resistance while maintaining a high resistance change with very small temperature differentials.